1. Field of the Invention
The present invention relates to a close-up lens, and in particular to a close-up lens with an internal focusing system having a constant total lens length, an imaging apparatus, and a method for focusing a close-up lens.
2. Related Background Art
A photo-taking lens for a close-up photograph has been proposed in, for example, Japanese Patent Application Laid-Open No. 2005-004041. Unlike an ordinary photo-taking lens, a close-up lens has to focus an object locating from infinity to a close distance allowing a life-size magnification or in the vicinity thereof, so that moving amounts of lens groups upon focusing necessarily become large. Accordingly, it is unsuitable for auto focus. When you try to focus from infinity to a close distance providing a life-size magnification by extending the whole lens system in a body, you have to move it a distance same as the focal length thereof. On this occasion, since variations in spherical aberration and the image plane cannot be suppressed, a lot of lenses have adopted a floating system in which moving speed of a portion of lens group differs from that of the whole lens system.
However, even a close-up lens with a floating system has still had a large moving amount of the lens group, so that it is unsuited for auto focus.